1. Field of the Invention
This invention relates to devices for the initiation of pyrotechnic, explosive, or propellant elements as are used in a wellbore and particularly to the safety of such devices both within and without the wellbore.
2. Background
The use of explosive devices within a wellbore can be traced to the early days of the petroleum industry when explosives, most notably, highly unstable nitroglycerin, were dropped free into wells to make the well "come-in" or produce. Use of explosives or propellants has, over the years, taken many forms, most notably: perforating; explosive fracturing; use of propellant (gas generator) powered devices for setting anchors or packers; tubing/casing cutting; and back-off tools. These systems must all be actuated or initiated by a deflagrating or detonating device whose function is controlled by a command action or signal. An initiating system consists of four basis elements: (1) a conveyance means to transport and locate the system within the wellbore; (2) a command path to send the firing signal; (3) an initiation charge (explosive, pyrotechnic, etc.) and (4) any safety controls or interlocks within the system. These initiating systems are generally of three types: percussive--a mechanical, impact-actuated device such as the primer in small arms ammunition; electrical--either a hot wire bridge (as the filament in a light bulb) or a spark-gap (as in an automotive spark plug); or an exploding wire or foil system in which an extremely high current is passed through the device causing a shock wave to be generated that is sufficient to cause sympathetic detonation of the initiating charge, operating similar to triggering devices utilized in nuclear devices.
Each of the above have operational problems that the present invention corrects or improves upon. Percussive systems utilize a primary, and hence sensitive, explosive as the initiator, that has a potential for sensitivity to mechanical shock. Electrically triggered systems require that wires be attached to the initiation charge, which wires can then act as antennae, making the system susceptible to accidental initiation from EMI (ElectroMagnetic Interference), RFI (Radio Frequency Interference), EMP (ElectroMagnetic Pulse), and ESD (ElectroStatic Discharge). To mitigate this potential it is not unusual for all other operations on and around the drilling rig to be curtailed or restricted during these operations. While exploding wire/foil systems are much less susceptible to accidental discharge from extraneous stimuli, they require substantial surface support and are inherently costly, thereby limiting their general usage in the industry.
The system of the present invention not only addresses the safety concerns set out above, both within and without the wellbore, by providing an enhanced insensitivity to both electric and mechanical shock stimuli, it also provides a simple to use system with attendant lower costs that make it more generally usable within the industry.